1. Field of the Invention
This invention relates generally to microwave power tubes and more particularly to linear-beam traveling wave tubes.
2. Description of the Prior Art
The traveling wave tube amplifier is widely used as a source of RF and microwave power for a variety of radar and communications applications. The conventional traveling wave tube, particularly in its low and medium power versions, in unmatched in bandwidth capability by either the klystron or the cross-field tube. A basic traveling wave tube amplifier consists of an electron gun which projects a focussed electron beam through a slow-wave structure (typically a helix or a coupled-cavity structure) toward a collector. The electrons are maintained in a narrow beam through the center of the slow-wave structure by a magnetic field. A c-w or pulse signal, coupled onto the slow-wave structure in the vicinity of the electron gun, generates a wave that travels in close proximity to the slow-wave structure at a velocity which is slightly less than the electron velocity. The electron velocity is, of course, determined by the potential difference between the electron gun cathode and the collector. The potential difference is adjusted to insure that the electrons, on the average, travel slightly faster than the RF wave. The electric field of the RF wave on the slow-wave structure interacts with the electric field created by the electron beam causing an increase in the amplitude of the RF wave on the slow-wave structure, thus producing the desired amplification.
Many traveling wave tubes utilize a helix wound with molybdenum wire as the slow-wave structure. The amount by which the input RF wave is slowed depends upon the pitch and radius of the helix. Interaction between the RF wave and the electron beam produces a bunching of electrons in the beam. This bunching processes, in turn, contributes to amplification of the RF wave.
The amplified RF wave is extracted from the helix at the downstream end of the tube. The electrons continue toward the collector where they are collected. Both theory and experiment indicate that some of the RF wave is reflected from the output end of the tube. The reflected wave may travel back through the helix to the tube input and cause unwanted oscillations. Consequently, it is customary to insert lossy materials such as carbon film at locations along the helix between the electron gun and collector to attenuate the reflected traveling wave. Unfortunately, this procedure attenuates the forward wave as well as the reflected wave. Another technique for reducing the reflected wave is to sever the helix. The sever prevents reflections from the load and the output terminal from reaching the input terminal. Although the forward growing wave on the input section of the helix is lost at the sever, current and velocity modulation remain impressed upon the electron beam which carries the signal across the sever region for further amplification in the output section.
The electrons which travel through the helix are collected in a collector at the downstream end of the tube. When the collector is at the same potential as the body of the tube, the electrons strike the collector at a relatively high velocity. Electron energy is then converted to heat at the collector surface. By reducing (depressing) the voltage on the collector below the tube body potential, the velocities of the electrons striking the collector and the heat generated in the collector are reduced. As a result, a depressed collector recovers some of the power in the spent electron beam.
A variety of articles are pertinent to an understanding of the present invention. One example is: Walter Beam, "On the Possibility of an Amplification in Space Charge Potential Depressed Electron Streams," Proc. IRE, pp. 454-462, Apr. 1955. The Beam article derives analytical expressions to predict the behavior of electron streams subject to space-charge effects. Another pertinent publication is: R. Hayes, "A Synchronous Wave Amplifier," IEEE Transactions on Electron Devices, March 1964, pp. 98-101. The Hayes article presents the derivation on analytical expressions for the gain of a linear tube device similar to a klystron. A further pertinent article is: T. Wesselberg, "A Thick Beam Analysis of Transverse Wave Propagation on Electron Beams," Proc. 4th International Congress on Microwave Tubes, pp. 657-663, Sept. 1962. The Wesselberg article presents a variety of dispersion and energy relations of transverse electromagnetic waves on electron beams. Another publication of interest is: B. Vural, "Double Stream Cyclotron Wave Amplifier," IEEE Transactions on electron Devices, Vol. ED-15, No. 1, January 1968, pp. 2-6. The Vural article discusses the operation of an electron beam amplifier utilizing a non-uniform magnetic field. Finally, another publication of interest is: M.R. Currie, et al., "The Cascade Backward Wave Amplifier: A High Gain Voltage Tuned Filter for Microwaves," Proc. IRE, pp. 1617-1631, Nov. 1955. The Currie et al article discusses an electron beam tube which utilizes two helices--an input helix and an output helix. The input signal is introduced at the collector end of the input helix and the signal is amplified as it travels toward the gun end of the tube. The amplified signal is dissipated in a matched helix termination at the gun end of the tube. However, the modulated electron beam passes through a drift tube and then into a second helix or other slow-wave structure also interacting with a backward wave. The output signal is then taken from the gun end of the output helix, with the collector end of the output helix matched. The article goes on to explain that such a wave has a group velocity and phase velocity in opposite directions.
A patent pertinent to an understanding of the present invention is U.S. Pat. No. 4,389,593, entitled "Active Dielectric Waveguide Amplifier or Oscillator Using a High Density Charged Particle Beam," issued to the present inventors. The patent discloses a circuitless particle beam device that eliminates the requirement for an internal slow wave structure. A circularly polarized RF wave propagates on a high density particle beam within an oversized waveguide and interacts with the beam to produce amplification.
In general, traveling wave tube dimensions are scaled by desired output wavelengths. At frequencies (wavelengths) in the millimeter wave range dimensions become extremely small and miniaturized slow wave structures and supports are used. As slow wave structures become smaller, heat dissipation problems become more severe. At millimeter wave frequencies, the slow wave circuit structure contributes a substantial amount to the cost and complexity of the tube.
Those concerned with the development of the power tubes art have consistantly sought inexpensive and simply constructed designs which are nevertheless capable of high power output.